another resistor killer

You need to figure in the heat capacity of the layer of alumina under the resistive layer.

Thin films are made of metal (so their thermal conductivity is higher than that of thick film resistive elements) and they are thinner than thick film resistive elements, so the heat doesn't have to travel as far.

Most resistor data sheets do have a curve for peak dissipation as a function of time. It doesn't sound as if you have looked at that.

Theorising usually involves thinking about what is going on in more detail than you seem to manage.

We blew out a box full of thickfilm attenuators testing this pulser, attens rated for 2 and 10 and 20 watts.

I'm thinking that the heterogenous structure of thickfilms creates microscopic hot points that a thinfilm doesn't have. Tiny sparks.

No, most don't. Show me one that goes into the region at issue here.

The AoE X-chapters book tortures a lot of resistors, around page 27. Have you read that? Fig 1x37, trace A, is close but it's one-shot and I want to run millions of pulses to be sure nothing degrades.

One experiment is worth a thousand expert opinions.

- Wernher von Braun

One stupid insult does not demonstrate competance.

- J Larkin

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Thin films are less than micron thick. A layer of metal just a single atom thick is only a few ohm per square, so thin film resistors are built up of a long meandering thin tracks.

Why did you keep on blowing them up? You should have been able to work out how distribute the energy over more devices.

Thick films depend on a mix of metal oxide and glass powders. The scale is coarser than thin film parts. Finding a data sheet wouldn't have been a better use of your time than theorising from a rather narrow knowledge base.

Learning how to spell competence would be the first step along that path.

john larkin wrote: <snip>

HV Resistors compact solid resistors should be bullet-proof for this app...

Would a non-inductively-wound wirewound resistor work well enough? You would have plenty of mass to average-out the pulse energy.

WWs are great for pulse overload, not so great for PCB density. The best would be to use three (or two, or one) surface-mount 1206 thickfilm that we have in stock.

I could stand a micohenry or so parasitic inductance. The 1.5K will in fact be in series with a small inductor.

I think testing parts is the only way to understand this.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

The data stops at 10 us, and the 1206 is only good for about 20 watts peak for continuous pulsing. The tests should be interesting.

Have you considered using your superb spell-checking skills in a new career? Third-grade substitute teacher comes to mind.

But you should get the its it's thing under control first.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

There's your answer; make the resistor and the inductor one and the same component. For a small investment in suitable machinery this gives you total security of supply, quality control and an edge over any competitor who can't make things but just buys them in (or tries to copy your design without realising what that component really does).

Vertical integration was the cornerstone of nearly all the successful electronics firms. (Philips even owned the sand quarries to supply the sand to make the glass to make the valves and light bulbs.)

Experiment with winding a number of turns of resistance wire on a former in one direction, then winding some more in the opposite direction. The ratio between the two sets of turns can be adjusted to give the required inductance and the total number of turns gives the resistance. The former could be a small piece of heatproof material shaped like a dog's bone to retain the wire, with a notch to catch the wire and prevent it from unwinding at the reversal point.

Yikes. That would be a huge diversion from getting a product done.

I found one paper that shows that thinfilms are tougher than thickfilms, but thinfilm MELFs are even better. That makes sense.

I'm not a fan of MELFs - they roll off ones bench onto the floor - but a boy has to do what he has to do.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Interesting, thanks.

I don't think I have room for those on this project. The PCB density is crazy and we don't have much height either.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

A cheap (0.6 cents) 1206 thickfilm is pulsing at 300 volts, 1 usec, 1 KHz. That's 180 watts peak.

After a few million pulses, resistance has changed not at all, to 5 digits. 498.72 ohms.

Should I run that for a few days, or crank up to 500 volts maybe?

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

When I visited the factory of a smart meter manufacturer I noticed that they used melf surface mount resistors for mains voltage sensing. There were several in series. John

Makes sense. For a given pcb footprint, they have about pi times the surface area to work with, for a correspondingly bigger conductor area. The cooling might be even better.

That would be interesting, to find some papers somewhere that discuss the benefits of melfs vs regular surfmount parts.

Menawhile, I'm seeing unexpected goodies blasting a cheap thickfilm

I don't think I have any melf resistors here. It would be interesting to break one in half and measure the ceramics, re thermal conductivity.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

One maybe silly question? On that board with big transistor and small resistors, why not make the transistor the 'dissipater'? So not a switch but add some simple electronics to limit it'''''s current ? Emittor resistor and fixed base voltage, current sense feedback, there are many way?

Zorry Englitch is not my lugguage Sjecked by SpellShaker

I agree with von Braun But you need experts too. he hired a few very good ones.

If the curves stay flat at pulse widths below 300usec, there's not a lot of point at looking at shorter pulse. Then heat doesn't diffuse out of the resistive track over shorter periods.

One 1206 is clearly not big enough to handle your job. Seven in parallel might do it, as I suggested above, but you don't seem to have thought about that.

There was a Review scientific Instruments paper many years ago, when some physicist published an elaborate multi-resistor network to act as terminator for 50R transmission line carrying a high voltage pulse.

The design didn't strike me as inspired when I read the paper, but it had been tested and seemed to work.

The spell-checking skill involved isn't mine. Thunderbird does it automatically/

When your editing program can do the spell checking for you, the career would be more like educating the dyslexic. One very clever - if dyslexic

- engineer I knew couldn't see the point of getting the spelling right - if the combination of letters he used sounded right it had to mean what he intended it to mean

I know the theory and mostly get it right, but we all make typo's. Your competance/competence confusion doesn't seem to be a typo - you consistently make that particular mistake with that word and several others like it.

This misses the point. The Vishay resistor data showed that - at least for their surface mount thin film resistors - the heat doesn't get beyond the resistive track itself for about 300usec.

If you get the track too hot for any time shorter than that it can melt (or at least get hot enough to let the atoms move around). For their resistors, nothing lower than 10k can take more that 1kV, which equates to a peak current of 100mA.

Once you've work out how much resistive area you need to use to work with any pulse shorter than 300usec, you then need work the duty cycle of your short pulses and make sure that you can dissipate the average power to ambient without getting the average temperature too high.

MELF resistors may have more surface area to dissipate heat over the long term but the peak short term power dissipation limit strictly determined by what happening in the resistive track itself.

Why not read the resistor data sheet. If they don't specify the performance for short pulses, the implication is that they aren't designed to cope with pulses current.

Not for your application. The ceramic has no effect on the short pulse limit.

The resistor will see a square voltage pulse, so that's what I'm attacking it with. And my little circuit was easy to make.

The 1r resistor is just a way to let me see the current, so I can get a heads-up warning if the DUT starts to change a lot. The real test is to pulse for a while and stop occasionally and measure the resistance, to see if the pulsing is changing anything.

What I'm seeing so far is zero change of resistance after many

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

In the newsgroup

alt.possessive.its.has.no.apostrophe

that error is referred to as "the greengrocer's apostrophe"

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I can remember an English teacher telling us that only a few percent of people could learn to use the apostrophe correctly.